The use of active noise cancellation in headsets has been utilized for some time. For example, U.S. Pat. No. 4,644,581 shows the use of a microphone and driver transducer in a headset designed to attenuate transmitted noise by counter noise. Langberg in U.S. Pat. No. 4,985,925 shows ANR in an earplug having multiple feedback loops.
Perhaps the first use of a speaker and microphone in an acoustically close volume was discussed and shown in Chaplin, U.S. Pat. No. 4,527,282.
Several other patents exist relating to the use of active noise cancellation in head sets. One example includes the work done by Bourk, U.S. Pat. No. 5,182,774, in which cancellation is achieved with the use of low, high, and mid range filters, and where the microphone is located coaxial with the driver (i.e., speaker). This invention differs from that device in several ways. Firstly, the circuitry includes a low pass filter and two band pass filters instead of mid and high pass filters. Secondly, this invention also includes a unique bridge circuit that bypasses the noise cancellation circuitry when the active system is turned off. Thirdly, this invention includes a booster circuit to compensate for low frequency losses when active noise cancellation is in operation. Fourthly, this invention does not require or implement coaxial location of the microphone element and the driver.